The present invention relates to a rotary magnetic head drum apparatus suitable for use in a helical scan-type magnetic recording and reproducing system.
As disclosed in Japanese Utility Model Unexamined Publication No. 59-89449, a conventional type of rotary magnetic head drum apparatus comprises an upper drum (rotary drum) provided with video heads and an lower drum (stationary drum) located in opposition to the bottom of the rotary drum. A magnetic tape is looped around a portion of the outer circumference of the drum apparatus, and tracks of recorded signals are formed on the magnetic tape in a direction oblique to the length thereof. In such a drum apparatus, the diameter of the rotary drum is larger than the diameter of the stationary drum, and a taper is formed around the circumference of a lower edge portion of the rotary drum such that the minimum diameter of the tapered lower edge portion is not smaller than the diameter of the stationary drum.
As shown in FIGS. 1, 2A and 2B, the taper is formed around the circumference of the lower portion 109b of a rotary drum 109 that opposes the top of a stationary drum 111. Windows (one of which is illustrated at 112) are formed in the tapered portion for allowing a portion of a video head 106 to project from the window 112. The video head 106 is fixed to a bottom end surface 109c of the rotary drum 109.
The advantage of such a drum apparatus as described above is that it is possible to prevent a thin magnetic tape of a high-density recording type from squeaking due to a sticky slip occurring between the tape and the stationary drum.
However, such a conventional type of drum apparatus gives no consideration to a stable contact (see FIG. 2B) between a magnetic tape 110 and an edge portion of the window 112 which is formed in the side surface of the rotary drum 109 and through which is projected a part of the video head 106 fixed to the bottom end surface 109c of the rotary drum 109. As a result, jitters may occur in synchronization with the cycle of drum rotation.